The next-generation multimedia wireless communication systems which are being actively researched are required to process various pieces of information, such as video and wireless data, in addition to the early voice-centered service with a higher data transmission rate. The 4th generation wireless communication systems which are now being developed, following the 3rd generation wireless communication systems, are aiming at supporting the high-speed data service of downlink 1 Gbps (Gigabits per second) and uplink 500 Mbps (Megabits per second). The object of the wireless communication system is to establish reliable communications between a number of users irrespective of their positions and mobility. However, a wireless channel has abnormal characteristics, such as path loss, noise, a fading phenomenon due to multi-path, inter-symbol interference (ISI), and the Doppler effect resulting from the mobility of a user equipment. A variety of techniques are being developed in order to overcome the abnormal characteristics of the wireless channel and to increase the reliability of wireless communication.
Technology for supporting reliable and high-speed data service includes Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), and so on. An OFDM system is being taken into consideration after the 3rd generation systems which are able to attenuate the ISI effect with low complexity. The OFDM system converts symbols, received in series, into N (N is a natural number) parallel symbols and transmits them on respective separated N subcarriers. The subcarriers maintain their orthogonality in the frequency domain. It is expected that the market for mobile communications will shift from the existing Code Division Multiple Access (CDMA) systems to OFDM-based systems. MIMO technology is used to improve the efficiency of data transmission and reception using multiple transmit antennas and multiple receive antennas. The MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. A MIMO channel matrix depending on the number of receive antennas and the number of transmit antennas can be decomposed into a number of independent channels. Each of the independent channels is referred to as a layer or a stream. The number of layers is referred to as a rank.
For the purpose of data transmission/reception, system synchronization acquisition, channel information feedback, or the like, there is a need to estimate an uplink channel or a downlink channel in a wireless communication system. In an environment of using the wireless communication system, fading occurs due to multi-path time delay. Channel estimation is a process of recovering a transmit signal by compensating for signal distortion which occurs when an environment changes rapidly due to fading. In general, the channel estimation is performed by using a reference signal (RS) known to both a transmitter and a receiver.
There are several methods proposed to extend the coverage of a base station (BS). One of them is a method of employing a relation station in the wireless communication system. For example, a relay station technique is one of main techniques for long term evolution (LTE)-advanced which is one of promising candidates of international mobile telecommunication (IMT)-advanced, i.e., a mobile communication system of a next generation (post-3rd generation).
The relay station is a device for relaying a signal between the BS and a user equipment (UE), and is used for cell coverage extension and throughput enhancement of the wireless communication system. An uplink and a downlink between the BS and the relay station are backhaul links, and an uplink and a downlink between the BS and the UE or between the relay station and the UE are access links. Hereinafter, a signal transmitted through the backhaul link is referred to as a backhaul signal, and a signal transmitted through the access link is referred to as an access signal.
In an LTE-advanced (LTE-A) system, an LTE-A UE and a legacy UE may be used together. In a subframe for the LTE-A, a radio resource for the LTE-A UE may be effectively used for various usages, whereas a radio resource for the legacy UE may not be used or may be easily wasted.
Accordingly, there is a need for a method for effectively utilizing resources for unused subframes.